Receptor activity modifying proteins (RAMPS) are a family of three proteins having approximately 31% identity. RAMPs are approximately 150-177 amino acid integral membrane proteins with a cleavable signal peptide, predominant amino-terminal (N-terminal) extracellular domain, one transmembrane spanning domain and a 9 amino acid intracellular carboxy-terminal domain. At least one RAMP is expressed in all cells known to express CRLR (a 7 TM GPCR). RAMPS associate with either calcitonin receptor-like receptor (CRLR) or calcitonin receptors (CTR) to influence their ligand affinity and specificity. Both CRLR and CTR belong to the B family of GPCRs. The N-terminal domains of this B family all contain multiple highly conserved cysteine residues suggesting the presence of conserved secondary structure by virtue of disulfide linkage necessary for binding to their homologous family of peptide ligands. The N-terminal fragment of the human parathyroid hormone receptor 1 (PTHR1) and Glucagon-like peptide 1 receptor (also members of the class B family of GPCRs) has been purified from bacteria, renatured and shown to bind their respective ligands with significant affinity. Because transmembrane proteins and especially 7 TM proteins are difficult to separate from their lipid environment and maintain their native conformation, the unique features of CRLR/RAMP interaction with CGRP may allow structural studies of the extramembranous N-terminal fragment of the CRLR to be a feasible and still fruitful compromise toward the understanding of the structural basis of ligand-receptor interaction. We are investigating whether the N-terminal extracellular domains of RAMP1 and CRLR are sufficient for their stable association and whether the heterodimer is capable of binding CGRP. Toward this goal, we are currently using a variety of expression systems to produce sufficient quantities of each N-terminal fragments of CRLR and RAMPs 1 and 2 for both structural and functional interaction studies.